The present invention relates in general to a cold shield assembly, and more particularly, to a lower cold shield aligned and fixed an upper cold shield while the cold shields are lowered to a cryogenic temperature.
Infrared (IR) imaging systems function on the basis that all objects constantly emit IR radiation as a function of their temperature. The radiated IR light may be picked up by sophisticated electronic equipment (e.g., thermal imaging devices) which converts the picked up IR to an electrical signal to be displayed on a video monitor, analyzed by computer or recorded on film. The electronic equipment that picks up the radiated IR light is capable of detecting differences in radiated IR light among different objects of a scanned scene.
Imaging applications in which IR imaging systems are used have created a need to increase the ability of sophisticated electronic equipment to detect minute irradiance differences in objects located in a scene. To this end, thermal imaging devices that are cryogenically cooled have been engineered and fabricated. These cryogenically cooled thermal imaging devices are currently available in the market. The advantage of such a device is increased resolution and sensitivity. For example, cryogenically cooled devices can detect temperature differences as low as 0.2° F. (i.e., 0.1° C.) from more than one thousand feet (300 meters) away.
Non-Imaging applications that use a single focal plane to view several fields can take advantage of the cooled focal plane technology. In these applications single focal planes can be divided by external or internal optics to split the field of view into two or more discrete locations on the focal plane. A robust example of this technology is one in which the cold shield incorporates the optical element that is responsible for dividing the fields.
To fabricate these cryogenically cooled devices, the lens assembly is typically aligned and fixed to the focal plane array at ambient temperature. The alignment therebetween must be maintained while the focal plane array and the lens assembly are cooled to cryogenic temperature levels. To this end, the lens assembly and focal plane array are aligned while at ambient temperature, cooled to cryogenic temperature levels and the alignment therebetween is checked. If the alignment therebetween is not satisfactory, then the focal plane array and the lens assembly are heated to ambient temperature, the focal plane array and lens assembly is repositioned with respect to each other based on the checked alignment. Thereafter, the focal plane array and the lens assembly are cooled to cryogenic temperature level and the alignment therebetween is checked again. The process is reiterated until the alignment between the focal plane array and the lens assembly is satisfactory.
Problems associated with this reiterative process are as follows. First, the time required to align the lens assembly and the focal plane array may be time consuming. In particular, each cycle of positioning, cooling, checking and heating to ambient temperatures may take hours. Second, the alignment accuracy obtainable through this process may be insufficient for certain applications which require even more perfectly aligned lens assembly and focal plane array than that which is achievable with the above-mentioned process. In particular, as stated above, the lens assembly and focal plane array are fixed to each other at ambient temperatures. The lens assembly and focal plane array are subsequently cooled to cryogenic levels and the alignment therebetween is checked. The problem with this process is that as the lens assembly and focal plane array are cryogenically cooled, these parts contract according to their coefficient of thermal expansion. This introduces variables in the alignment process which are inherent thereto and may not be eliminated.
Accordingly, there is a substantial need to provide a new method by which the lens assembly may be aligned to the focal plane array with a greater accuracy than the alignment obtainable when the lens assembly is fixed to the focal plane array at ambient temperatures.